1. Field of the Invention
The present invention relates to a surface modified expanded polytetrafluoroethylene material, and methods used to produce it.
2. Description of Related Art
Expanded polytetrafluoroethylene (PTFE) products are widely employed today in a variety of devices, including filters, fabrics, implantable sheets, and vascular grafts. Expanded PTFE is formed by heating and rapidly expanding a PTFE precursor material to form a microstructure of polymeric nodes interconnected by polymeric fibrils with microscopic void spaces therebetween. Expanded PTFE can be formed in the manner described in U.S. Pat. Nos. 3,953,566, 3,962,153, 4,096,227, and 4,187,390, all incorporated by reference.
The relative dimensions of the node and fibrils can be tailored to affect both the porosity and surface properties of expanded PTFE. For example, a highly porous surface can result from the creation of long, widely spaced, substantially parallel fibrils. A highly porous surface is often beneficial in certain applications. For example, a high degree of porosity may enhance lamination bond strength or improve certain filter performance properties. In specific medical applications, more open structure may be beneficial since the structure can encourage tissue ingrowth and attachment. On a flat sheet, however, there is a limit to the amount of ingrowth structure that can be achieved through only manipulation of nodes and fibrils during expansion. To further enhance tissue ingrowth beyond that of a highly porous surface, a roughened or textured surface is believed to be required. Thus, in medical applications requiring rapid tissue ingrowth, an ideal expanded PTFE surface may have both a high degree of porosity and further surface modification to provide some degree of macroscopic texturing.
In addition to enhancing the rate of tissue attachment, a textured surface may be desirable in other applications, for instance to enhance bond strength, abrasion, heat transfer, optical, or other properties. Increased roughness may also be desirable to increase surface friction, flow turbulence, sound abatement, or exposed surface area.
Various methods of altering the surface properties of expanded PTFE have been suggested in the past. For example, an expanded PTFE surface treatment process is taught in U.S. Pat. Nos. 5,462,781 and 5,437,900 to Zukowski. Disclosed is a plasma treatment process that removes fibrils to a selected depth to leave freestanding nodal ridges. These freestanding nodes are easily bent or deflected due to the lack of supporting fibrils. Such a treated surface affects the hydrophobicity, bondability, and appearance, but may not necessarily elicit an optimum tissue response due to an excessively “soft” exposed surface.
U.S. Pat. Nos. 4,550,447 and 4,647,416 to Seiler, Jr., et al., teach a PTFE surface treatment that creates full density PTFE ribs on the outer surface of an expanded PTFE tube. Although this process may increase macro-roughness by the producing stiff ridges, the ridges are unexpanded and are thus non-porous. Non porous unexpanded ridges are believed to be undesirable since they can achieve only minimal tissue attachment and ingrowth.
U.S. Pat. Nos. 4,332,035 and 4,713,070 to Mano, teach an expanded PTFE process wherein differential heat is applied to opposing surfaces of a tubular wall. The process results in a randomly oriented ridge and valley texture. The ridges comprise node groupings having interconnecting fibrils. The valleys have long fibrils, which interconnect the node groupings. Although this process may increase surface “roughness” due to the relatively stiff ridges, the valleys remain soft due to the long interconnecting fibrils. Thus the valleys produced by the Mano process contribute little to the overall macro-roughness of the device and therefore probably do not enhance the rate of tissue attachment.
For specific applications, it is believed desirable to generate a highly porous expanded PTFE surface having optimum roughness or texturing. A ridge and valley surface texture can enhance the macro-roughness. However, it is believed desirable that ridges are both relatively stiff and at least somewhat porous. Similarly, it is desirable that the valleys remain porous yet provide some addition means to contribute to the macro-roughness of the device. The ridges and valleys would ideally have a pattern that could be controlled and tailored for specific applications.